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Mind the Gap—for Expansion, That Is

Quality Matters

Mind the Gap—for Expansion, That Is

The expansion gap between dissimilar materials is a critical detail to prevent costly callbacks and warranty headaches 

By Tim Kampert August 23, 2019
Providing an expansion gap between the window and masonry wall is a construction best practice.
Just leave a 1/8-inch gap between the brick and the window, then fill it with a backer rod and high-quality sealant to absorb expansion and contraction. (Photo: courtesy IBACOS)
This article first appeared in the September 2019 issue of Pro Builder.

Some callbacks are so obvious and easily preventable that they should never happen. Yet they do. A perfect example is the improper detailing of masonry or natural stone veneer around windows and doors. 

In many of the homes I see during field inspections, brick or stone veneer has been butted tight against window or door trim or frames. It may look good, but the problem is that trim materials, usually wood or vinyl, expand when temperatures and humidity rise, while brick and stone are much more stable in those conditions. When the trim or frame expands and pushes against the brick, something must give, and it’s not going to be the brick.

In many cases, the expanding trim or frame has no choice but to move away from the brick, which squeezes the window or door, causing it to bind. At that point, the builder’s only real option to relieve the pressure and keep it from happening again is to either cut the brick (in the case of a brick return) or to shave back the trim, if possible. Either way, it’s an expensive fix that most builders would rather avoid.


Construction best practices related articles from Professional Builder's sister site

Leave a Gap Between Materials

Prevention is remarkably simple: just leave a 1/8-inch gap between the brick and the window, then fill it with a backer rod and high-quality sealant that meets ASTM C920 Class 25 minimum standards to absorb expansion and contraction.

Getting this right isn’t difficult and, in fact, masonry manufacturers and suppliers include shims in shipments to the jobsite that are sized to ensure a proper gap between their materials and other, dissimilar, products, namely windows and doors. 

In the past, most masons knew to use those shims. But the exodus of skilled labor from the construction trades has left a workforce that doesn’t understand a lot of small but important details, including that one.

The key question is how to get today’s workers to do the job right—and consistently.


Expansion gap between materials
A right-sized gap between masonry veneer and other materials—filled with the appropriate sealant—is a small but crucial detail. The effort required to ensure it’s done right pays for itself many times over. (Photo: courtesy IBACOS)


Training Trades for Performance

Training your trades for this detail is straightforward. Once you point out the shims (or other scrap material on site that happens to be appropriately sized for that job) and explain what to do with them (and why), most crews understand it easily and quickly. 

A complicating factor is the high labor turnover in the masonry trades and the fact that you can’t count on getting the same crew for every house ... which means you have to keep an eye on it and maintain diligent training.

The builders I know who get consistently good results tend to pair training with inspections for accountability. For instance, the builder’s quality assurance manager or job supervisor might make it a point to check each house while masonry veneer is being applied. If the veneer is butted tight against a dissimilar material, the masonry sub has to pull it off and redo it, at their expense. It usually doesn’t take long before the sub’s managers are emphasizing this detail to their crews.

To be fair to everyone, builders need to include this gap in their written specifications or scopes of work and make their expectations and consequences clear to the masonry sub, as well as the trade that will apply the backer rod and sealant (usually the painter). But, of course, that’s the same requirement for any successful quality assurance effort. 


Tim Kampert drives quality and performance in home building as a building performance specialist of the PERFORM Builder Solutions team at IBACOS.


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